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The following procedure requires the use of a distributor wrench or a skinney arm with an open end wrench and a timing light.
The vehicle emission control decal, which is found underhood, will often contain specifications or procedures for checking and adjusting timing that have been updated during production. The information contained on the label should always be used if it differs from these instructions.
On most HEI systems, the tachometer connects to the TACH terminal on the coil and to a ground. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Also, note that some tachometers won't work at all with an HEI system. Consult the tachometer manufacturer if the instructions supplied with the unit do not give the proper connection.
WARNING Never ground the HEI TACH terminal; serious system damage will result, including ignition module burnout. Set the parking brake and block the drive wheels, then warm the engine to normal operating temperature.
Shut the engine OFF and connect the a timing light to the No. 1 spark plug (right front 2.8L engine, left front 4.3L engine or front plug on in-line engines):
If using a non-inductive type, connect an adapter between the No. 1 spark plug and the spark plug wire; DO NOT puncture the spark plug wire, for this will allow a voltage arc.
If using an inductive type, clamp it around the No. 1 spark plug wire.
If using a magnetic type, place the probe in the connector located near the damper pulley; this type must be used with special electronic timing equipment.
Do not under any circumstances pierce the insulation of a spark plug wire in order to connect the timing light.
Fig. 1: A common timing mark scale
Clean off the timing marks, then label the pulley or damper notch and the timing scale with white chalk or paint for better visibility. If the timing notch on the damper or pulley is not visible from the top, the crankshaft should be bumped around using the starter or turned using a wrench on the front pulley bolt, in order to bring the mark to an accessible position.
On early model vehicles not equipped with EST, disconnect and plug the vacuum advance hose at the distributor to prevent any distributor vacuum advance. The vacuum line is the rubber hose connected to the metal cone-shaped canister on the side of the distributor. A short screw, pencil, or a golf tee can be used to plug the hose.
Fig. 2: Aim the timing light at the mark and scale, but be careful not to contact the fan or drive belts
Fig. 3: Timing marks are often difficult to see; this engine is equipped with a magnetic probe holder which may be used with special equipment
On later model vehicles equipped with EST, the electronic spark timing must be disabled or bypassed to prevent the control module from advancing timing while you are attempting to set it. This would obviously lead to an incorrect base timing setting. There are 2 possible methods of disabling the EST system, depending on the type of engine:
On 2.5L engines, ground the "A" and "B" terminals on the ALDL connector under the dash before adjusting the timing.
On all other engines using the EST distributor, disengage the timing connector wire. On a few of the earlier model vehicles, the 4-terminal EST connector must be disengaged from the distributor. Most later model vehicles are equipped with a single wire timing bypass connector. On these later model vehicles the bypass wire is usually a tan wire with a black stripe. This wire usually breaks out of the wiring harness conduit adjacent to the distributor, but on some vehicles it may break out of a taped section just below the heater case in the passenger compartment.
Start the engine, then check and adjust the idle speed, as necessary. The idle speed must be properly set to prevent any centrifugal advance of timing in the distributor.
Aim the timing light at the timing marks. Be careful not to touch the fan, which may appear to be standing still. Keep your clothes and hair along with the timing light's wires clear of the fan, belts and pulleys. If the pulley or damper notch isn't aligned with the proper timing mark, the timing will have to be adjusted.
TDC or Top Dead Center corresponds to 0° mark on the scale. Either B, BTDC, or Before Top Dead Center, may be shown as BEFORE on the scale, while A, ATDC or After Top Dead Center, may be shown as AFTER.
Loosen the distributor base clamp locknut. You can buy special wrenches which make this task a lot easier on certain models. Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation (found in the firing order illustrations earlier in this Section) to retard, and against the direction to advance.
Once the timing is properly set, hold the distributor to keep it from the turning and tighten the locknut. Check the timing again after finishing with the nut in case the distributor moved as you tightened it.
If applicable, remove the plug and connect the distributor vacuum hose.
If necessary check and/or adjust the idle speed.
Shut off the engine and reconnect the EST wire (if equipped), then disconnect the timing light and tachometer.
Your timing should be set to 10 deg. BTDC for automatic or 6 deg.BTDC for manual.